Precision structures are susceptible to disturbances that can produce structural vibrations. Since precision structures, such as precision truss structures for space, bridge, or building applications, tend to have little inherent damping, these vibrations can lead to serious performance degradation. An efficient means of adding damping to large precision structures in a controlled manner is of considerable importance. Both active and passive damping techniques have been utilized. However, active systems suffer from high cost, low reliability, and poor low-level or threshold performance. On the other hand, passive damping systems require no power, are often less expensive than active damping devices and do not drive the structure unstable. Thus, passive damping systems have proven to play a significant role in the overall design of large precision structures.
There are several manners of implementing passive damping in a structure such as a truss structure. Two of the more common methods are viscoelastic damping and viscous damping. Both such methods can be incorporated into strut-like elements for use in truss structures. Viscoelastic damping methods have been in widespread use in vibration absorbing applications and are often applied in panel-like structures. These devices, though capable of providing isolation or damping for most applications, exhibit cyclic wear and excessive sensitivity to temperature and are susceptible to other environmental conditions.
Viscous dampers include a fluid reservoir sealed in a damping structure which utilizes viscous fluid sheer forces to provide damping. One particular viscous damper is described in U.S. Pat. No. 4,760,996 to Davis issued Aug. 2, 1988 and assigned to Honeywell Inc. The viscous damper described therein achieves common axis and which is attached to an end piece and a base at opposite ends of the shaft; the shaft maintaining a fixed separation distance therebetween. A piston having an axial bore hole and a flange extending therefrom for coupling to a load is positioned about the shaft in a coaxial relationship forming a fluid annulus between the piston and the shaft. A first and second bellows are positioned in axial alignment with the shaft and are fluidly sealed at opposite ends of the bellows by the end piece and the base, respectively, and the flange to form two fluid chambers therein. A fluid gap couples the fluid chambers in the first and second bellows that are formed between the piston, inner walls of the bellows and the flange extension from the piston. This arrangement obtains damping by purely viscous fluid sheer forces. As the load attached to the flange moves, the volume of one chamber increases while the volume of the second decreases. The overall volume, due to the fixed distance between the end piece of the first bellows and the base piece of the second bellows maintained by the shaft, remains constant. Thus, fluid of constant volume that is contained within the two chambers and the gap is distributed to the chambers in accordance with the movement of the load providing a damping function.
A viscous damper such as described in U.S. Pat. No. 4,760,996 has several associated problems. The damping potential of such a viscous damper is partly determinable by the axial compliancy of the bellows. The pressure in the chambers of the viscous damper is determined by the fluid in the chambers and is distributed variably to the bellows in accordance with the velocity of the load. The expansion of the bellows due to the pressure reduces the fluid shear forces through the fluid annulus with a resultant loss in damping. Thus, the ability to dampen large loads and/or handle loads of high velocity is diminished. In order to prevent the bellows from expanding in a manner to increase their volume, or in other words volumetric expansion, when pressure is increased in a chamber, the bellows must be kept relatively stiff. Because of the necessary stiffness, due to the potential pressure in the chamber being relatively high, a decrease in the relative possible stroke along the axis of the viscous damper results. In addition, in order to keep the fluid gap continually retained between the piston and the shaft to prevent contact and friction therebetween, the bellows must also be made relatively stiff to prevent the bellows from extending in a radial direction due to volumetric expansion. Such stiffness once again decreases the stroke potential of the viscous damper.
A viscous damping technique which offers high damping for truss structures is the D-Strut.TM. as described in "Viscous Damped Space Structure for Reduced Titter," by J. F. Wilson and L. P. Davis, 58th Shock and Vibration Symposium, August 1987. The D-Strut.TM. which is used to replace a nominal type strut in a truss structure includes a small viscous damper placed in series with an inner tube and the damper and inner tube are placed in parallel with an outer tube. An axial displacement across the strut produces a displacement across the damper. The damper forces a fluid through a small diameter orifice causing a shear in the fluid and providing viscous damping for the structure. The damper is basically two compliant cavities connected by the small diameter orifice. The compliancy of the cavities reduces the shear forces when a pressure of the fluid in the compliant cavities causes the cavity volume to change. Thus, a resultant loss in damping occurs. However, the D-Strut.TM. provides much higher damping capabilities.
As indicated above, there are various problems with regard to both viscoelastic damping devices and viscous devices. Therefore, there is a need to provide isolation and damping which can withstand environmental conditions, application of cyclic forces and which is structured such that minimal pressure is applied to flexible portions of the viscous damping structure so as to allow for maximum stroke capability and/or to afford large load or high velocity capability.